Method of resistance welding using pulsed dc superimposed over steady state dc voltage

ABSTRACT

Apparatus for welding, particularly resistance seam welding, wherein the voltage applied across the weld comprises a steady state direct current voltage having superimposed thereon an alternating current voltage or a chopped direct current voltage. This provides a true diffusion weld, results in a substantially uniform grain structure across the weld, and enables extremely high welding speeds.

United States Patent Inventor Kenneth E. Opal Oakmont, Pa. Appl. No. 13,695 Filed Feb. 24, 1970 Division of Ser. No. 766,533, Oct. 10, 1968, Pat. No. 3,521,025. Patented Mar. 23, 1971 Assignee Power Control Corporation Pittsburgh, Pa.

METHOD OF RESISTANCE WELDING USING PULSED DC SUPERIMPOSED OVER STEADY STATE DC VOLTAGE s 4 Claims, 3 Drawing Figs.

US. Cl 219/59, 2l9/l08, 219/1 13, 219/131 Int. Cl 823k 3/06 Field of Search 219/59, 64,

67, I08, 131, 113; 323/22 (SCR) [56] References Cited UNITED STATES PATENTS 2,021,477 1l/l935 Bohn 2l9/108X 2,776,399 l/l957 Sommeria 2l9/l08X 3,249,735 5/1966 Needham 219/131 3,361,892 l/l968 Spencer 219/1 35X 3,459,920 8/1969 Sevenco 219/131 Primary Examiner-J. V. Truhe Assistant Examiner-L. A. Schutzman Attorney-Brown, Murray, Flick & Peckham ABSTRACT: Apparatus for welding, particularly resistance seam welding, wherein the voltage applied across the weld comprises a steady state direct current voltage having superimposed thereon an alternating current voltage or a chopped direct current voltage. This provides a true diffusion weld, results in a substantially uniform grain structure across the weld, and enables extremely high welding speeds.

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42 28 H FIRING Fmma CIRCUIT cmcu/r CONTROLLER 24 From Rectifier 20 c 0 4000A- 3000" mvewron. From Rectifier 20 8 KENNETH E. OPAL p 0 I J J4 I Attorneys ME'lillllQD h lltESlSTANCE WELDENG USllNG PULSED 1M1 SUPERKMPUSED (WEE STEADY STATE lilC VGLTAGE CROSS-REFERENCES TO RELATED APPLICATIONS Copending application Ser. No. 766,533, filed Oct. 10, 1968, now US. Pat. No. 3,521,025, issued Jun. 21, 1970 of which this is a division.

BACKGROUND OF THE INVENTION While not limited thereto, the present invention is particularly adapted for use in the welding of can bodies. in the past, most cans had been formed from tin plate sheets which are formed into a cylindrical shape and the two edges joined along a seam formed by overlapping, reversely bent portions. This seam is then soldered and tinned; and in modern can-forming equipment, the entire process of joining, soldering and tinning can be achieved at a production rate of up to 500 cans per minute.

Recently, it has been found that plain black plate, instead of tin plate, can be coated with various resins to provide satisfactory can bodies. instead of forming a seam by overlapping reversely bent portions which must then be soldered and tinned, the side seam in a black plate can body can be formed by welding overlapping edges of a flat sheet formed into a cylinder. This results in a small but highly significant savings in metal. While welded can bodies have many advantages, they have not found wide acceptance to date, the reason being that the production rate of such cans is lower than soldered cans because of limitations of existing welding equipment.

in order to successfully weld can bodies at high speeds, currents on the order of thousands of amperes are required. If current is applied to the welding head in the form of a steady direct current, a satisfactory weld may be produced at one end of the can; but at the other end, burning of the metal may result. This is believed to be due to propagation of heat from one end of the can to the other as the seam is being welded, resulting in too high a temperature and resultant burning at the trailing end.

in an effort to overcome these inherent problems of high speed seam welding, proposals have been made to chop the extremely high currents required at high frequencies, above 1,000 cycles per second. This proposal would theoretically eliminate the problem of heat propagation and resultant buming incurred with a steady state direct current voltage. However, it is virtually impossible to chop extremely high currents at the frequencies required. For example, the inherent line inductance and time constant of the system limit the rate at which switching can occur at high current levels; and, in any event, the chopping equipment required for such applications is extremely expensive. 7

SUMMARY OF THE lNVENTlON As an overall object, the present invention seeks to provide new and improved seam welding apparatus which achieves higher speeds and better quality welds than any known welding apparatus.

More specifically, an object of the invention is to provide welding apparatus of the type described wherein a high frequency, chopped direct current of relatively low amplitude is superimposed on a steady state direct current of greater amplitude. r

in accordance with one embodiment of the invention, overlapped metal edges are passed between electrodes, a stead state direct current of relatively high amplitude is impressed across the electrodes which is sufficient to raise the temperature of the metal to a point slightly below its melting temperature, and high frequency direct current pulses are superimposed on the steady state direct current, the pulses being of sufficient magnitude to raise the temperature of the metal to its fusion temperature and thus produce along the seam a line of contiguous weld points.

hi this manner, it is necessary to chop direct currents of relatively low amperages only; however when these are superimposed on the steady state direct current which itself will not cause welding, a weld of exceptionally good quality is produced at extremely high line speeds.

As will be understood, the frequency of the direct current pulses should increase as the speed of the workpiece being welded increases. At workpiece speeds of about 3 feet per second, for example, the frequency of the pulses should be about 1.6 kilocycles to give a weld spot spacing of approximately 40 weld spots per inch. As the speed of the workpiece decreases, however, so also can the frequency of the pulses to maintain the same weld spacing.

The apparatus of the invention preferably comprises an inverter which produces a square wave direct current output, the output of the inverter being doubled in a circuit similar to a full-wave rectifier and applied across the welding electrodes. Also applied across the electrodes is the steady state direct current which is preferably about 3 times the magnitude of the pulses. An inductive choke in the line between the steady state direct current source and the welding electrodes serves to isolate it from the pulse source.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FlG. l is a perspective view showing the manner in which a can body is welded along a seam;

H6. 2 is a schematic diagram of the welding apparatus of the present invention; and

FlG. 3 illustrates waveforms appearing at various points in the circuit of H0. 2.

With reference now to the drawings, and particularly to FIG. 1, the upper portion of a can body ill is shown comprising plain carbon black plate which is the name normally given to tin plate prior to tinning. The present invention is concerned with apparatus for welding the overlapped ends 112 of the can body formed from black plate. in the formation of the can body, a sheet of black plate, No. 29 gauge or thinner, is initially wrapped around a mandrel to form a cylindrical shape with the ends of the sheet overlapped as at 12. Thereafter, the overlapped ends must be passed between the opposite electrodes 14 and 16 of a resistance welder at speeds as high as 3 feet per second. The welding electrodes M and 16 may comprise a pair of wheels, the lower wheel l6 being carried on the mandrel around which the can body is formed.

The welding apparatus itself is shown in HS. 2 and includes the two welding electrodes l4 and 16 connected through an inductive choke 18 to a rectifier 2f the output of the rectifier being smoothed or filtered by means of capacitor 22. The power for the rectifier 20 is supplied from a source of threephase alternating current 24. The current source 24 is connected to a controller, such as a Variac (Trademark); while the controller is connected through a three-phase transformer 23 to the rectifier 20.

The circuitry also includes an inverter 30 of the semiconductive controlled rectifier type, the rectifiers in the inverter Fill being controlled by means of a conventional firing circuit 32. The inverter converts the direct current output from a rectifier 34 into a chopper or square wave output appearing as waveform A in MG. 3. Waveform A is applied through transformer 36 to a circuit 3% similar to a full-wave rectifier, but without an outputsmoothing capacitor. The circuit 38 includes a pair of silicon controlled rectifiers 45b and 52 having their anodes connected to the opposite ends of the secondary winding of transformer 36, and their cathodes connected to the upper welding electrode 114. The lower welding electrode 116, in turn, is connected to a center tap on the secondary winding of transformer 36. The gates of the silicon controlled rectifiers ill and 42 are connected to a controllable firing circuit 44 such that the point at which the rectifiers fire during each half cycle of the applied waveform A can be controlled. in this manner, the power supplied across the welding electrodes l4 and 16 can be controlled by varying the firing angles of rectifiers 40 and 42 through firing circuit 44. The circuit is completed by means of diodes 46 and 47 which provide a current path to prevent arcing between the electrodes 14 and 16 when the electrodes leave the end of a can body. With a welding potential established across electrodes 14 and 16 with the polarity shown, the rectifiers 46 and 47 are not conducting because the potential across electrodes 14 and 16 is less than the contact potential of the two diodes 46 and 47 in series. However, when the can body leaves the welding electrodes and prior to making contact with the next can body, the energy stored in inductor 18 will be discharged through diodes 46 and 47 rather than across the electrodes 14 and 16, thereby minimizing arcing. The purpose of the inductor 18 is to isolate the rectifier 20 from the high frequency pulses produced at the output of circuit 38. In a circuit where the frequency of the pulses from circuit 38 is about 1.6 kilocycles, the inductance of inductor 18 should be about 40 microhenries.

In order to produce satisfactory welds at speeds of 3 feet per second, it is necessary to provide output pulses from circuit 38 at a frequency above 1,000 cycles per second, preferably about 1.6 kilocycles By virtue of the frequency doubling characteristics of the circuit 38, the output frequency from inverter 30 need be only 800 cycles per second for the example given. When doubled in circuit 38, therefore, the resulting pulses appearing as waveform B in FIG. 3 will have a frequency of 1.6 kilocycles. In the particular waveform given in FIG. 3, the firing angle of the silicon controlled rectifiers 40 and 42 is 90 and 270, respectively. However, by increasing or decreasing the firing angle, the width of the pulses in waveform B can be correspondingly increased or decreased as well as the power supplied across the electrodes 14 and 16.

The voltage appearing across the electrodes M and 16 appears as waveform C in FIG. 3. The voltage from rectifier 20 is at level of plus one volt; and superimposed on this are the pulses from waveform B which have a magnitude of about 0.3 volt. This produces a current wave shape across the electrodes illustrated as waveform D in FIG. 3 wherein current pulses of approximately L000 amperes are superimposed on a steady state current of about 3,000 amperes supplied from the rectifier 20. From waveform D, it can be seen that by virtue of the fact that the pulses superimposed on the base current are of relatively low amplitude, the time constant of the circuit due to inherent line inductances will permit the 1,000 ampere pulses to rise to their maximum levels at frequencies above 1,000 cycles per second. This would not be the case, for example, where an attempt is made to chop the entire 4,000 amperes.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention. In this respect, it will be apparent that an alternating current could be superimposed on the direct current, rather than a chopped direct current, with equal effectiveness.

I claim:

1. Apparatus for welding edges of a metallic workpiece, comprising a source of steady state direct current, means for applying said source of steady state direct current across a pair of resistance welding electrodes between which said edges are positioned, means for producing relative movement between said edges and said electrodes, an inverter for producing a square wave output, rectifier means coupled to said inverter for producing a train of direct current pulses, and means for applying said pulses across said electrodes in parallel with said means for applying said source of steady state direct current.

2. The apparatus of claim 1 wherein said rectifier means comprises a full-wave rectifier incorporating semiconductive controlled rectifiers as unidirectional current devices.

3. The apparatus of claim 2, including means for varying the points during each cycle of said square wave at which said semiconductive controlled rectifiers fire.

4. The apparatus of claim 1 including an inductor connecting said rectifier to said electrodes. 

1. Apparatus for welding edges of a metallic workpiece, comprising a source of steady state direct current, means for applying said source of steady state direct current across a pair of resistance welding electrodes between which said edges are positioned, means for producing relative movement between said edges and said electrodes, an inverter for producing a square wave output, rectifier means coupled to said inverter for producing a train of direct current pulses, and means for applying said pulses across said electrodes in parallel with said means for applying said source of steady state direct current.
 2. The apparatus of claim 1 wherein said rectifier means comprises a full-wave rectifier incorporating semiconductive controlled rectifiers as unidirectional current devices.
 3. The apparatus of claim 2, including means for varying the points during each cycle of said square wave at which said semiconductive controlled rectifiers fire.
 4. The apparatus of claim 1 including an inductor connecting said rectifier to said electrodes. 